How do Ecto bindings work?

Elixir is not directly using what you wrote for the ecto query, but ecto transforms the AST of your code into something that makes sense to elixir.

A macro doesn’t execute the code passed in, It gets an AST of the code and it can then do whatever it wants. I haven’t looked at how Ecto actually does this, I am assuming that it gets a hold of the AST and transforms it into a SQL query based on the operators that are being used and then it executes the code. One way to inspect it is by looking at the struct that was returned:

iex(7)> q = from(u in User, where: u.id == 3, order_by: [asc: :id]) |> Map.from_struct
%{
  assocs: [],
  distinct: nil,
  from: {"users", UA.Accounts.User},
  group_bys: [],
  havings: [],
  joins: [],
  limit: nil,
  lock: nil,
  offset: nil,
  order_bys: [
    %Ecto.Query.QueryExpr{
      expr: [asc: {{:., [], [{:&, [], [0]}, :id]}, [], []}],
      file: "iex",
      line: 7,
      params: []
    }
  ],
  prefix: nil,
  preloads: [],
  select: nil,
  sources: nil,
  updates: [],
  wheres: [
    %Ecto.Query.BooleanExpr{
      expr: {:==, [],
       [
         {{:., [], [{:&, [], [0]}, :id]}, [], []},
         %Ecto.Query.Tagged{tag: nil, type: {0, :id}, value: 3}
       ]},
      file: "iex",
      line: 7,
      op: :and,
      params: []
    }
  ]
}

Here is an example that should give you more clarith around macros, the expression 1 + 2 is never evaluated:

defmodule M do
  defmacro add({:+, _, [op1, op2]}) do
    quote do
      "Add #{unquote(op1)} and #{unquote(op2)} yourself!"
    end
  end
end

defmodule Test do
  def test do
    require M
    M.add(1 + 3)
  end
end

Test.test() |> IO.puts
# => Add 1 and 3 yourself!